The invention relates to metallic glasses.
It has long been desirable to engineer metallic materials that possess the properties of strength and durability without paying the penalty of excessive weight. To overcome the problems of mechanical failures due to dislocations and grain boundaries and to produce desirable magnetic and electrical properties, materials scientist have experimented with producing metallic glasses through the rapid quenching of thin streams of materials. The ribbons produced are typically no greater than 0.5 mm thick and have many desirable properties (mechanical and magnetic), but they are not bulk materials (i.e. having as-cast configuration) exhibiting bulk properties.
It has been about 30 years since Paul Duwez and colleagues demonstrated that metallic glasses from the melt can be produced using his "gun technique" if the quench rate was sufficiently rapid (e.g. .about.10.sup.6 K/S) (See P. Duwez, R.H. Willens, and W. Klement, Jr., J. Appl. Phys. 31, 135 (1960)). Since that time much experimental and theoretical work has disclosed the conditions necessary to produce and maintain the metallic glass ("amorphous") state. David Turnbull has been among the leaders in the field. His work in the late 40's with metal alloy (mercury) drops and that of Vonnegut with oxide coated tin drops demonstrated that the undercooling of metallic materials followed a path similar to non-metallic materials. (See D. Trumbull, J. Appl. Phys. 20, 817 (1949) and B. Vonnegut, J. Colloid Sci. 3, 563 (1948)). Deep undercoolings were possible if heterophase nucleants were either absent or neutralized. Even relatively large samples (e.g. a few grams) could be undercooled if nucleants were removed by appropriate fluxing techniques.
In order for a glass to form, the melt must reach the glass forming temperature, T.sub.g, before crystal nucleation can occur. The material must thus undercool below the liquidus temperature, T.sub.l, in order to reach T.sub.g. The reduced glass temperature ratio T.sub.rg =T.sub.g /T.sub.l becomes an important parameter.
The homogeneous nucleation rate decreases with increased T.sub.rg showing the desirability of choosing metallic liquids for which T.sub.g is as close to T.sub.l as possible.
Liquid metals, unlike non-metallic liquids, require negligible thermal activation for the nucleation of crystallization, thereby making glass formation difficult, if not impossible without the addition of impurity admixtures that are chosen to necessitate a redistribution by partition or local reordering thus preventing crystallization in time periods for which the viscosity of the melt can approach 10.sup.13 poise. These admixtures of impurities play another important role, namely, stabilizing the resultant glass against subsequent crystallization and substantial recalescence.
Among the techniques for producing metallic glasses from the melt are: splat quenching, melt spinning, and melt atomization -- all depending on a large surface area in order to achieve a high quench rate. Alternately, amorphous materials have been produced by sputtering, vapor deposition, and self-substitute quenching. These techniques are reviewed by H.H. Liebermann, in "Sample Preparation: Methods and Process characterization," Amorphous Metallic Alloys, ed. Luborsky, Butterworth Monographs in Materials, 1983, pp. 1-7. In the ion implantation approach, effective substrate quench rates of 10.sup.14 K/s have been achieved. This surface modification technique allows for the surface treatment of metals, providing surfaces with the potential for high corrosion resistance.
Metallic glass applications also include mechanical property improvements (hardness, fracture strength, ductility, toughness). Yet with the present restrictions on sample thickness -- in ribbons and foils -- there is relatively low resistance to cycle fatigue under tension. The ribbons have found important application in electrical transformer core windings and motors because of their soft magnetic behavior and low magnetic hysteresis losses, with a world potential energy savings of perhaps 2.times.10.sup.10 kWk per year.
One of the major problems with the present state of the art in metallic glass formation is in producing bulk, as-cast configurations. Amorphous metal powder consolidations are one potential solution to this problem, but suffer from many of the traditional shortcomings of composite materials. The present invention offers an alternative approach, namely, producing metallic "foams", i.e., open solid structures that may possess glass properties, low density, and the ability to take on bulk (as-cast) configurations.